Japan has summoned China’s ambassador to protest an alleged violation of what Tokyo regards as its territorial waters after a Chinese navy ship sailed close to disputed islands in East China Sea for the first time.

The Japanese Defense Ministry said later that the Chinese frigate had left the waters and sailed to the north.

A Japanese Defense Ministry official also said that three Russian military vessels had been seen in the waters around the disputed islands at around the same time.

The official said an investigation was underway to see whether the Chinese and Russian moves were in anyway related.

“The fact that (China) sent a naval ship to the contiguous waters of our Senkaku Islands for the first time is an act that unilaterally increases tension and our nation is gravely concerned,” Japanese Chief Cabinet Secretary Yoshihide Suga said on Thursday.

The uninhabited East China Sea islands lie about 220 km (135 miles) west of Taiwan and are controlled by Tokyo but claimed by Beijing which calls them Diaoyu.

A group of disputed islands, Uotsuri island (top), Minamikojima (bottom) and Kitakojima, known as Senkaku in Japan and Diaoyu in China is seen in the East China Sea. (file photo)

Tension between China and Japan escalated in 2012 when the Japanese government nationalized control over three of the Senkaku Islands in the East China Sea.

The uninhabited yet strategically-important island group has been under the Japanese administrative control since the reversion of Okinawa to Japan from US administrative rule in 1972.

China maintains that the islands are an inherent part of its territory and that it has indisputable sovereignty over them. The Japanese government, instead, regards them as a part of its Ishigaki Island.